This invention pertains to metallurgy, and more specifically to separating out heavy metal ions from a solution.
In order to recover silver from photographic processing solutions the prior art has caused such solutions to flow through a tank, say from top to bottom only, and radially through convolutions of zinc-coated window screen contained within the tank. Through simple galvanic cell action the silver displaces the zinc on the screen. The silver in the form of a sludge is later recovered from the bottom of the tank.
Experimental procedures have been undertaken for the absorbtion of silver by ion-exchange resins. This has been in connection with bleach-fix solutions, as "Regeneration of Ferric-EDTA-Thiosulfate Bleach-Fix Solution by Anion-Exchange Resins", H. Iwano et al, (Japan), "Journal of Applied Photographic Engineering", Vol. 2, No. 2, pgs. 65-69, Spring 1976.
This procedure has been for a spent bleach-fix solution containing 4.32 grams of silver per liter, a relatively very large amount.
The paper concludes with the statement:
"The new regeneration system presented here still has some problems that have to be solved before introduction to practical applications but it may provide an improvement for pollution abatement." PA1 "I. a. (4) Operation of an ion exchange process for removal of silver at concentrations of 0.05 gm/l or less requires over ten times the quantity of resin as found by Buyers (ref. 3); this quantity is probably impractical for Air Force applications. Therefore, the ion exchange process should be evaluated at fix bath silver concentrations of between 0.05 gm/l and 0.50 gm/l." PA1 "b (1) Inorganic anion exchange resin fouling resulting from repetitive silver stripping from spent photographic fix solutions may have been caused by the electrostatic neutralization on the resin exchange sites of colloidal silver sulfide and elemental sulfur which are formed by the acid induced decomposition of thiosulfate and silver thiosulfate ions. Other colloids would neutralize exchange sites in a similar manner." PA1 "c. Ion Exchange, Rinse Water Process PA1 Removal of silver from rinse water by ion exchange appears to be feasible from a theoretical standpoint; however, the ability to directly discharge rinse water following ion exchange treatment will depend largely upon the limits of silver removal attainable with the ion exchange unit together with discharge standards imposed upon Air Force facilities. Lower limits of silver removal by ion exchange have not been established as yet." PA1 "5. At a given moment the manufactured uses of ion exchanging resins for the absorbtion of silver from washing waters is still unprofitable because of the high cost of anionite, just as the filters, saturated with silver, must be burnt. The wide instillation of ionites for seizing silver from the washing waters gets propogation or lowers their cost, or when deciding on the problem of seizing silver without burning the resin."
Another experimental procedure was performed in Britain, utilizing 200 millileters of resin in a 23 millimeter diameter tube.
Further prior art efforts include a paper approved for public release; distribution unlimited, by the Air Force Weapons Laboratory, New Mexico, entitled "Development and Application of Ion Exchange Silver Recovery System", by A. Buyers et al, May 1974.
The Conclusions contain certain information, such as:
Still further efforts were reported in the Russian paper from "TexHuka KuHo u TeAceaudeHuA," 1962, 6, pp. 59-62; in which Conclusion 5, as translated, reads:
It is known in the trade that an ion exchange silver recovery system was installed in a commercial film-developing laboratory. However, the resin packed together and the flow of water was reduced. This caused a backup of the outflow water from the processing machine and spoiled film being processed, so that the recovery system was summarily removed from the plant.
It appears from all of the above that the prior art had not reached a workable method or apparatus, such as has been achieved in the present invention.